Many communications standards are unable to effectively use the entire potential throughput of the channel specified in the standard due to the time-varying nature of that channel. To provide a reliable level of information flow, the data encoding and modulation is chosen to be relatively conservative. That is, even during a “bad” interval, information can still be effectively passed/communicated. However, it is often the case that the channel is much better than the pessimistic assumption, and more data could be reliably passed given knowledge about the quality of the channel.
Current communications standards are capable of adapting to channel variations, but only crudely. The most common approaches require the transmitter and receiver to make measurements, analyze the data, agree to change the modulation/coding used, and finally to synchronize the change. This is often a slow process that is ineffective in dealing with rapidly changing channel environments.